Polarized Electron Beams and Polarimetry · 1978, SLAC - polarized ~e ... 1 Spin Physics History of Spin Physics Modern Studies in Spin Physics 2 Polarized Electron Beams Source of

Outline

Polarized Electron Beams and Polarimetry

E.Chudakov1

1Hall A, JLab

JLab Summer Detector/Computer Lectureshttp:

//www.jlab.org/∼gen/talks/polar lect.pdf

http://www.jlab.org/~gen/talks/polar_lect.pdf

http://www.jlab.org/~gen/talks/polar_lect.pdf

Outline

Outline

1 Spin PhysicsHistory of Spin PhysicsModern Studies in Spin Physics

2 Polarized Electron BeamsSource of PolarizationElectron Beam Polarimetry

Outline

Outline



Spin Physics Polarized Electron Beams

Motivation

Polarized beams and targets are used to study various effectsof Spin Physics.

Present MotivationStudying the structure of composite particles (protons etc.)Studying the dynamics of strong interactionsSearch for “new physics”


Outline




History of Spin Physics

Before 1925 - atomic spectra ⇒ concepts of QuantumMechanics1925-1928 - Completion of QM with the concept of spinup to now - measuring anomalous magnetic moments1956-1957 - parity violation in weak interactions1955 - ... polarized proton beams1975 - ... polarized muon beams1975 - ... polarized electron beams1987 - ... proton spin puzzle


Introduction of Spin

Ralph de LaerKronig

early 1925 -introduced spin(unpublished)

Wolfgang Pauli1924-1925 -

exclusion principle1927 -

non-relativisticformalism

George SamuelUhlenbeck Goudsmit

late 1925 - introduced spin


Relativistic Description of Spin

Paul Adrien MauriceDirac

1927 - equation forelectron

Dirac’s Equation

Relativistic equation for fermions s = 12 , 3

2

Introduction of antiparticlesMagnetic moment of fermionsBasis for QED


Outline




Magnetic Moments of Pointlike Fermions

Magnetic moment: µ = g q2m · S ,

where:q, m, S are the particle’s charge, mass and spin,g - gyromagnetic factor.

Dirac’s equation with EM interaction for pointlike S = 12 : g = 2

Experimental values for (g − 2)/2

e−, e+: ≈ 0.001µ−, µ+: ≈ 0.001p : ≈ 1.79 - compositen : ≈ −0.91 - composite


Anomalous Magnetic Moments of Pointlike Fermions

Interactions of EM field with itself and with vacuumCalculable in QED

�

γ

q− q−

�

γ

q− q−

�

γ

f

f

q− q−

Corrections to (g-2)/2

0 (απ) · 0.5 ∼ (α

π)2 · ( m

mf)2

Precise measurements vs detailed calculationsSubstructureElectron: test for “pure” QEDMuon: search for “new physics”: existence of heavyparticles (via loops), anomalous couplings etc.


Muon Magnetic Moment

CERN (1960-1970), BNL 1990-2000 spin precessionexperiments

p + A → .. + π− → µ− + νµ ,µ - polarized!~µ− → e− + νµ + νe,~pe ≈ along Sµ

Precession ∆ω = g−22

eBmc

10

10 2

10 3

10 4

10 5

10 6

10 7

0 50 100 150 200

32-233 µs

233-467 µs

467-700 µs

Time [µs]N

umbe

r of

Pos

itro

ns/1

50ns 32-82 µs

650-700 µs

World average:aexper

µ − atheorµ = 22(10) · 10−10

Remarkably accurate!


Interactions with Spin: Symmetries

1 2 3 4R ⇒ •⇐ R L ⇐ •⇒ L L ⇐ •⇐ R R ⇒ •⇒ L

Parity-conjugated L ⇐ •⇒ L R ⇒ •⇐ R R ⇒ •⇒ L L ⇐ •⇐ R

Rotational symmetry - 4 potentially different processesParity conservation: σLL = σRR, σLR = σRL

Parity is non-conserving in weak interactionsTypical Experiments with Polarization

Beam, target polarizations Pb, Pt ,Beam polarization is flipped periodically

Parity violatingEx: Pb 6= 0, Pt = 0Measured:A = NL−NR

NL+NR= Pb

σL−σRσL+σR

Parity conservingEx: Pb 6= 0, Pt 6= 0Measured:A = NLL−NRL

NLL+NRL= PbPt

σLL−σRLσLL+σRL


Parity Violation in Electron Scattering

Møller Scattering

�γ

e−

e−

e−

e−

�γ

e−

e−

e−

e−

�Z

e−

e−

e−

e−

�Z

e−

e−

e−

e−

A = NL−NRNL+NR

= PbσL−σRσL+σR

σL−σRσL+σR

∝ GF Q2(1− 4sin2θW )

A ∼ 10−3 − 10−6 - very small!

1978, SLAC - polarized ~e− beam1978 ~e−D scattering (DIS) ⇒ sin2θW

2003, SLAC ~e−e− Møller scattering⇒ sin2θW

1989-2006 ~e−p/He elastic ⇒strange formafactors (Bates, JLab,Mainz)


Deep Inelastic Scattering

CERN (1987), muon beam “Proton Spin Crisis”:

Quark contribution to proton spin ∆q ∼ 15% against∼ 75% expected from quark modelsStudied at CERN, SLAC, DESY, JLab with polarizedmuon/electron beams and polarized target


Outline




How to Make Polarized Electrons?

Two methods are currently used:

Linac’s injectors: polarized photoeffectStorage rings: gradual polarization by Sokolov-Ternofeffect


Polarized Electron Guns

• High polarization > 50%• High current > 100µA• Fast polarization flipping > 10Hz• Stability against the helicity flip

GaAs guns• GaAs wafer• Optical pumping by circularly

polarized light• Electron extraction into vacuum

from the Cs and O2 treatedsurface

• Super-lattice cathodes P > 85%


Electron Gun at JLab

∼ 100 kV electron extractionLifetime depends on the full charge extractedQE decreaseVery high vacuum needed (ion bombardment kill thecathode)Laser spot can be moved to recover QE


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Requirements for Polarimetry

In precise PV experiments the systematic error is dominated bypolarimetry!

Typical systematics error > 1.5%

In some special cases ∼ 0.5%

Requirements for future experiments < 0.5%


Methods Used for Electron Polarimetry

Spin-dependent processes with a known analyzing power.Atomic Absorption

~e − ∼50 keV decelerated to ∼13 eV~e −(13eV ) + Ar → ~Ar∗ + e−, ~Ar∗ → Ar + (hν)σ

Atomic levels: (3p54p)3D3 → (3p64s)3P2 811.5nm fluorescencePotential σsyst ∼ 1%. Only relative so far, invasive, diff. beam

Spin-Orbital InteractionMott scattering, 0.1-10 MeV: e − ↑ + Z → e− + Z σsyst ∼3%,⇒1% (?) invasive, diff. beam

Spin-Spin Interaction• Møller scattering: ~e − + ~e − → e− + e− at >0.1 GeV,

σsyst ∼ 3-4% , ⇒ 0.5% Mostly invasive, diff. beam• Compton scattering: ~e − + (hν)σ → e− + γ at >0.5 GeV∼ 1-2%, ⇒0.5%. non-invasive, same beam


Mott Polarimetry0.1-10 MeV: e − ↑ + Au → e− + Au Analyzing power ∼ 1-3%:

• Nucleus thickness: phase shifts• Spin rotation functions• Electron screening, rad. corr.• No energy loss should be allowed• Single arm - background• Extrapolation to zero target

thickness

• e − ↑ < 5 µA - extrapolationneeded

JLab: σ(P)/P = 1%(Sherman)⊕ 0.5%(other) (unpublished) ⊕σ(extrapol)


Compton N↑↑−N↑↓N↑↑+N↑↓

= A · PbPt Møller~e − + (hν)σ → e− + γ QED.

• Detecting the γ at 0 angle• Detecting the e−

• Strong dAdk ′

- good σEγ/Eγ

• A ∝ kE at E < 20 GeV• T ∝ 1/(σ · A2) ∝ 1/k2 × 1/E2

• Plaser ∼ 100%• Non-invasive measurementSyst. error 3→50 GeV:∼ 1. → 0.5%

~e − + ~e − → e− + e− QED.

A(E) = −79

σlab ∼180 mb

ster

• Detecting the e− at θCM ∼ 90◦

• dAdθCM

|90◦ ∼ 0 - good systematics• Beam energy independent• Coincidence - no background• Ferromagnetic target PT ∼ 8%

Syst. error ∼ 3% typically, (0.5%)


Compton Polarimeters: Best Accuracy at High Energy

SLAC SLD

Beam:45.6 GeV3.5 · 1010e−×120 Hz ∼ 0.7 µA

Laser:• 532 nm, 50 mJ at 7 ns × 17 Hz• Crossing angle 10 mrad

Detector:• e− 17-30 GeV gas Cherenkov• γ detector - calorimeter

Stat: 1% in 3 minσ(P)/P

source SLD ILC1998 Goal

Laser polarization 0.10% 0.10%Analyzing power 0.40% 0.20%Linearity 0.20% 0.10%Electronic noise 0.20% 0.05%total 0.50% 0.25%

M.Woods, JLab Polarimetry workshop, 2003


Møller Polarimeter with Saturated Iron foil

JLab, Hall C, M. Hauger et al., NIM A 462, 382 (2001)

• External BZ ∼ 4 T• Target 4-10 µm, perp. to beam• Pt not measured• Important: annealing, etc.

source σ(A)/Aoptics, geometry 0.20%target 0.28%Levchuk effect 0.30%total 0.46%⇒ 100 µA ?

Tests for high current• Beam σX ≈ 50µm

> r = 12µm• At 20µA -

accidentals/real≈0.4• σstat ∼ 1% in 2h

20 µs

Kick (1-2 mm)

100 Hz to 10 kHz

Current Studies• A 1µA thick half-foil• Higher duty factor


Summary

Physics with polarized electron beamsStudying the structure of composite particles (protons etc.)Studying the dynamics of strong interactionsSearch for “new physics”

FeaturesPolarization ∼ 80%

Polarimetry σP ∼ 1%

Polarized Electron Beams and Polarimetry · 1978, SLAC - polarized ~e ... 1 Spin Physics History of Spin Physics Modern Studies in Spin Physics 2 Polarized Electron Beams Source of

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